The major goal of this project is to determine how the magnitude, pattern and growth of hearing loss and structural damage are altered as the physical characteristics of the stimulus (frequency intensity, duration, and scheduling) are varied. Another goal is to identify the behavioral and anatomical correlates of presbycusis in aged, but unexposed animals. These animals will then be used to study the potential interactions between age and noise exposure. These behavioral and anatomical studies share the long-term goal of identifying the relations between functional deficits and injury in the inner ear rather than specifying damage-risk criteria for human exposures. In addition, acoustic measures will be made from ear canals of noise- exposed chinchillas. These measures will provide physical indicators of cochlear integrity in the intact animal and information about nonlinear and active processes in the normal and damaged cochlea. Finally, evaluation of data available from industrial hearing conservation programs will be evaluated. One objective of the study will be to establish an appropriate control population of nonoccupationally exposed individuals who are otherwise unscreened for comparison to groups of workers exposed to occupational noise. The other objective is to identify factors which contribute to hearing loss observed in industrial populations, such as recreational noise exposure, smoking, military history, etc. Some of the experimental animals will be trained by food-reward, operant conditioning techniques so that measures of auditory function (e.g., pure- tone thresholds, frequency DL's etc.) can be obtained before, during, and after exposure to noise. HIstological evaluation of the specimens will include detailed study of plastic-embedded flat preparations so that counts of missing sensory and supporting cells can be made and cytocochleograms prepared for all specimens. Selected regions of the organ of Corti will be sectioned and examined by light and electron microscopy and analyzed morphometrically.